The present invention relates to a pressure control device for a pressure accumulator, in particular, for automotive vehicles with a brake slip control system. The pressure accumulator is fed by a disconnectible pump and at least two user components are adapted to be connected to the pressure accumulator, the user components requiring different minimum feed pressures for correct operation.
The problems with such a feeding of different pressure fluid user components are that the admissible lowest accumulator pressure has to be determined according to the highest minimum feed pressure of all connectible user components. Consequently, the pump feed will be constructed such that the pressure in the pressure accumulator is not allowed to drop under the level of the highest minimum pressure required. As a result, the pump feed has to be switched on relatively often, although the user component with the highest minimum feed pressure is put into operation only rarely.
Pressure control systems of the aforementioned type are described in the book by Dr.-Ing. Erwin Samal "Outlines of Practical Control Technology", published by R. Oldenbourg, 1967. On page 135, a pressure control valve is shown switching on and off the pump feed dependent on the pressure prevailing in a pressure accumulator. As another example, a water level regulator is shown on page 137 controlling the height of a water level by using an electric switching arrangement such that the pump feed is switched on upon attainment of a presettable minimum amount and the water level increases up to a presettable maximum amount, upon attainment of which the pump feed is switched off.
Likewise, pressure switches are shown in the handbook "Vickers Handbook of Hydraulics", 1966, in which a hydraulic fluid acts on a piston movable against the force of a spring and actuating a microswitch after having travelled a predetermined distance. However, pressure switches of this kind are only able to be used for switching on or off a unit so that such a switch would have to be available in duplicate, if a greater hysteresis were desired. Hydraulic switches with adjustable minimum switching pressure and adjustable maximum switching pressure have been described on pages 96 and 97.
The prior art switching elements mentioned hereinabove are not in all cases in a position to satisfactorily solve the problem of pressure control. Particularly in the event of several, different user components being connected to one single pressure accumulator, it is desirable to control the pressure in the pressure accumulator such that in each case only the minimum feed pressures of the connector user components are guaranteed. Especially with regard to brake slip control systems, pressure fluid being pressurized as constantly as possible is required for governing the pressure modulation. At the same time, however, the pressure accumulator feeds a hydraulic auxiliary force for actuation of the brake unit and maybe still other hydraulic user components. As to the hydraulic brake booster, the controlled delivery of pressure is not subjected to such strict conditions of maintaining constrant values as is required in the case of pressure fluid serving for control of the pressure modulation for the brake slip control system.